This invention relates to steam turbines, and more particularly, to a system and method for reducing turbine blade overheating and consequent distress that occurs because of turbine windage heating following trips of reheat turbines.
Following a turbine "trip", i.e., when high pressure steam from a boiler to a turbine impulse chamber is suddenly shut off, the steam flow in a high pressure (HP) element of a single stage reheat turbine can reverse in less than one second. By impulse chamber is meant a zone either ahead or immediately after a first stage in which a drain is located. The reason for such reversal is that closure of steam interceptor valves keeps the HP exhaust pressure at an elevated level, while the pressure at the HP inlet decays because of leakage around the turbine shaft and flow removal through the moisture drain system. In a double reheat turbine, the same situation also occurs between reheats.
It is well known that when there is reverse or negative steam flow, windage heat generation is higher with normal forward rotation of the turbine blades than with reverse or negative rotation. In the case of normal forward flow, windage heating in lower with forward positive rotation than with reverse, negative rotation. With respect to reverse flow and forward rotation, the flow capability is poorer because the flow is entering the blade passage from the wrong direction and the flow area is decreasing rather than increasing as the flow traverses the blade path from normal exhaust to inlet.
It has also been established that the highest windage losses during reverse flow and forward or normal rotation occur when the flow follows the suction or convex side of the blade passages. This phenomenon, in which the flow follows the passage wall that diverges from the flow direction, has been called the Coanda effect. (Normal forward flow typically tends to follow the wall that turns into the flow and the concave boundary or pressure side of the blade passage.) Occurrence of the Coanda effect during reverse flow conditions further increases losses by increasing windage heating.
The conventional solution to the problem of windage heating has involved heat removal by supplying sufficient ventilating steam to control the temperature so that blade distress does not occur. However, it is very difficult to evaluate forward rotation with reverse flow conditions, and despite detailed investigations of this problem, temperature predictions based on calculations extrapolating forward rotation, forward flow data have been overly conservative. The uncertainty of the analysis becomes successively greater with each stage that the steam passes through, as each stage adds some increment of incorrect temperature increase to the temperature of the preceding stage. These overly conservative predictions have resulted in designs in which the ventilating or drain valves supplied on ventilation systems are larger than they probably need be, at increased cost. Prevention of reverse flow would reduce the windage heating problem, thereby reducing the requirements and costs of the ventilation system, and would allow more accurate predictions of windage heating using the experimental data already availabe for a forward rotation, forward flow regime.
Accordingly, it is an object of this invention to reduce windage heating of turbine blades by controlling the direction of steam flow and eliminating the Coanda effect after a turbine trip.